Poly-lactide or polylactic acid (PLA) is a synthetic aliphatic polyester derived from renewal resources, such a corn, sugar beet and cassava, which can ultimately be degraded under composting conditions. Polymers such as PLA have received particular attention, notably for partly replacing certain articles and mouldings in polyvinyl chloride (PVC), which despite their good balance of properties have certain shortcomings.
Although attempts have been made to utilize PLA for various end-use applications, PLA is known to be brittle and exhibit low toughness, which can result in low impact strength products or articles. In addition, when damaged, PLA items exhibit an irreversible weakness due to the cicatrices.
Impact resistance of PLA can be modified by using existing polymeric impact modifiers; however, currently available polymeric impact modifiers always decrease transparency of PLA comprising material. Liquid plasticizer can be used at high content (>15%) to improve impact resistance of PLA, however during the life time of the PLA blend, there is migration and leaching of the plasticizer.
Impact modifiers such as rubber, poly(ethylene glycol) (PEG), and acrylonitrile-butadiene-styrene copolymer (ABS) have been tested. Nevertheless, the immiscibility between these impact modifying additives and the PLA matrix is a major drawback.
Commercially available BioStrength® 150 a methyl methacrylate-butadiene-styrene co-polymer (MBS) is one of the best currently available impact modifiers for PLA; however haze of the resulting PLA material increases from 5, for pure PLA to 95 when 15% w/w of BioStrength® 150 is added.
Plasticizers are additives that increase the fluidity of a material. Commonly used plasticizers, are tributyl citrate (TBC) and acetyl tributyl citrate (ATBC). However, when 15% TBC or ATBC are mixed with PLA, the present inventors observed a plasticizer migration after storage for a few days at room temperature in summer time (25-30° C.).
In turn, Grinsted plasticizer is said not to migrate, however the present inventors observed whitening of PLA-containing Grinsted plasticizer during storage. Additionally differential scanning calorimetry (DSC) showed beginning of crystallization on aged material. Therefore, it can be said that this material is not stable during longer period of time.
There is therefore a need to improve the compositions of the prior art. There is therefore a need to develop polymer compositions obtained from renewable resources having improved impact resistance and allowing objects to be manufactured by the usual techniques of plastics processing.